The present invention generally relates to the preparation of semiconductor grade single crystal silicon which is used in the manufacture of electronic components. More particularly, the present invention relates to single crystal silicon ingots and wafers having a first axially symmetric region of silicon self-interstitial dominated material which is substantially free of agglomerated intrinsic point defects and a second axially symmetric region, or core, of vacancy dominated material which is also substantially free of agglomerated vacancy defects, as well as a process for the preparation thereof.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (xe2x80x9cCzxe2x80x9d) method. In this method, polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
In recent years, it has been recognized that a number of defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Such defects arise, in part, due to the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects, which are known as vacancies and self-interstitials. Silicon crystals grown from a melt are typically grown with an excess of one or the other type of intrinsic point defect, either crystal lattice vacancies (xe2x80x9cVxe2x80x9d) or silicon self-interstitials (xe2x80x9cIxe2x80x9d). It has been suggested that the type and initial concentration of these point defects in the silicon are determined at the time of solidification and, if these concentrations reach a level of critical supersaturation in the system and the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur. Agglomerated intrinsic point defects in silicon can severely impact the yield potential of the material in the production of complex and highly integrated circuits.
Vacancy-type defects are recognized to be the origin of such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques such as Scanning Infrared Microscopy and Laser Scanning Tomography. Also present in regions of excess vacancies are defects which act as the nuclei for ring oxidation induced stacking faults (OISF). It is speculated that this particular defect is a high temperature nucleated oxygen agglomerate catalyzed by the presence of excess vacancies.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
The density of such vacancy and self-interstitial agglomerated defects in Czochralski silicon is conventionally within the range of about 1*103/cm3 to about 1*107/cm3. While these values are relatively low, agglomerated intrinsic point defects are of rapidly increasing importance to device manufacturers and, in fact, are now seen as yield-limiting factors in device fabrication processes.
To date, there generally exists three main approaches to dealing with the problem of agglomerated intrinsic point defects. The first approach includes methods which focus on crystal pulling techniques in order to reduce the number density of agglomerated intrinsic point defects in the ingot. This approach can be further subdivided into those methods having crystal pulling conditions which result in the formation of vacancy dominated material, and those methods having crystal pulling conditions which result in the formation of self-interstitial dominated material. For example, it has been suggested that the number density of agglomerated defects can be reduced by (i) controlling v/G0 to grow a crystal in which crystal lattice vacancies are the dominant intrinsic point defect, and (ii) influencing the nucleation rate of the agglomerated defects by altering (generally, by slowing down) the cooling rate of the silicon ingot from about 1100xc2x0 C. to about 1050xc2x0 C. during the crystal pulling process. While this approach reduces the number density of agglomerated defects, it does not prevent their formation. As the requirements imposed by device manufacturers become more and more stringent, the presence of these defects will continue to become more of a problem.
Others have suggested reducing the pull rate, during the growth of the body of the crystal, to a value less than about 0.4 mm/minute. This suggestion, however, is also not satisfactory because such a slow pull rate leads to a reduced throughput for each crystal puller. More importantly, such pull rates lead to the formation of single crystal silicon having a high concentration of self-interstitials. This high concentration, in turn, leads to the formation of agglomerated self-interstitial defects and all the resulting problems associated with such defects.
A second approach to dealing with the problem of agglomerated intrinsic point defects includes methods which focus on the dissolution or annihilation of agglomerated intrinsic point defects subsequent to their formation. Generally, this is achieved by using high temperature heat treatments of the silicon in wafer form. For example, Fusegawa et al. propose, in European Patent Application 503,816 A1, growing the silicon ingot at a growth rate in excess of 0.8 mm/minute, and heat treating the wafers which are sliced from the ingot at a temperature in the range of 1150xc2x0 C. to 1280xc2x0 C. to reduce the defect density in a thin region near the wafer surface. Such a process can be problematic, however, as the diameter of the ingot increases; that is, the growth rate may not be sufficient to ensure the material is entirely dominated by vacancy-type intrinsic point defects. If agglomerated silicon self-interstitial defects are formed, thermal treatments are generally not effective for removing or dissolving them.
A third approach to dealing with the problem of agglomerated intrinsic point defects is the epitaxial deposition of a thin crystalline layer of silicon on the surface of a single crystal silicon wafer. This process provides a single crystal silicon wafer having a surface which is substantially free of agglomerated intrinsic point defects. Epitaxial deposition, however, substantially increases the cost of the wafer.
In view of these developments, a need continues to exist for a method of single crystal silicon preparation which acts to prevent the formation of agglomerated self-interstitial intrinsic point defects by suppressing the agglomeration reactions which produce them, while affording the means by which to subsequently remove agglomerated vacancy defects present. Such a method would also afford single crystal silicon wafers having epi-like yield potential, in terms of the number of integrated circuits obtained per wafer, without having the high costs associated with an epitaxial process.
Among the objects of the present invention, therefore, is the provision of single crystal silicon ingot, or wafer derived therefrom, having an axially symmetric region of substantial radial width which is substantially free of defects resulting from an agglomeration of crystal lattice vacancies or silicon self-interstitials; the provision of such a wafer wherein a substantially defect free axially symmetric region of interstitial dominated material surrounds a core of vacancy dominated material; the provision of such a wafer wherein the core of vacancy material has a nonuniform distribution of agglomerated vacancy defects; the provision of a process for preparing a single crystal silicon ingot, or wafer derived therefrom, wherein concentration of self-interstitials is controlled in order to prevent an agglomeration of such defects in an axially symmetric segment of a constant diameter portion of the ingot, as the ingot cools from the solidification temperature; and, the provision of such a process wherein agglomerated vacancy defects, if present, are dissolved by thermal treatment of the silicon in wafer form.
Briefly, therefore, the present invention is directed to a single crystal silicon wafer having a central axis, a front side and a back side which are generally perpendicular to the central axis, a central plane between the front and back sides, a circumferential edge, and a radius extending from the central axis to the circumferential edge. The wafer comprises a first axially symmetric region, extending radially inwardly from the circumferential edge, in which silicon self-interstitials are the predominant intrinsic point defect and which is substantially free of agglomerated interstitial defects, and a second axially symmetric region in which vacancies are the predominant intrinsic point defect, the second axially symmetric region comprising a surface layer extending from the front side toward the central plane and a bulk layer extending from the surface layer to the central plane, the concentration of agglomerated vacancy defects present in the surface layer being less than the concentration in the bulk layer.
The present invention is further directed to a process for preparing a single crystal silicon wafer which is substantially free of agglomerated intrinsic point defects. The process comprising thermally annealing a single crystal silicon wafer at a temperature in excess of about 1000xc2x0 C. in an atmosphere of hydrogen, argon or a mixture thereof, said wafer having a central axis, a front side and a back side which are generally perpendicular to the central axis, a central plane between the front and back sides, a circumferential edge, a radius extending from the central axis to the circumferential edge, a first axially symmetric region extending radially inward from the circumferential edge in which silicon self-interstitials are the predominant intrinsic point defect and which is substantially free of agglomerated interstitial defects, and a second axially symmetric region, located radially inward of the first axially symmetric region, in which vacancies are the predominant intrinsic point defect. The thermal anneal acts to dissolve agglomerated vacancy defects present in the second axially symmetric region within a layer extending from the front side toward the central plane.
The present invention is still further direction to a process for preparing a silicon wafer which is substantially free of agglomerated intrinsic point defects, the wafer being sliced from a single crystal silicon ingot having a central axis, a seed-cone, an end-cone, and a constant diameter portion which extends between the seed-cone and the end-cone, the constant diameter portion having a circumferential edge and a radius extending from the circumferential edge toward the central axis, the ingot being grown from a silicon melt and then cooled from the solidification temperature in accordance with the Czochralski method. The process comprises (i) growing the single crystal silicon ingot, wherein the growth velocity, v, and an average axial temperature gradient, G0, are controlled during the growth of the constant diameter portion of the ingot over a temperature range from solidification to a temperature of no less than about 1325xc2x0 C. to cause the formation of a segment of the constant diameter portion which, upon cooling of the ingot from the solidification temperature, comprises a first axially symmetrical region extending radially inward from the circumferential edge toward the central axis in which silicon self-interstitials are the predominant intrinsic point defect and which is substantially free of agglomerated interstitial defects, and a second axially symmetric region in which vacancies are the predominant intrinsic point defect; (ii) slicing the segment of the constant diameter portion to obtain a wafer, the wafer having a front side and a back side which are generally perpendicular to the central axis, and a central plane between the front and back sides, the wafer comprising the first and second axially symmetric regions; and, (iii) thermally annealing the wafer at a temperature in excess of about 1000xc2x0 C. in an atmosphere of hydrogen, argon, oxygen, nitrogen, or a mixture thereof to dissolve agglomerated vacancy defects present in the second axially symmetric region within a layer extending from the front surface toward the central plane of the wafer.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.